Basically…

Pokemon from Gens 3~7 (2000s to early 2019) can only be brought forward using a system that will no longer be officially supported at some point.

Pokemon in Gens 8 and 9 (the Switch games) are mostly considered “current”, and can be swapped around between any current game and Pokemon Home.

Pokemon Home costs money to store Pokemon.

Generally, moving Pokemon between generations involves transferring them using some special tool. The final destination (unless you want to pay money) is usually a game, because that’s where you can actually use them. The games also have plenty of storage that’s “free” (at least, not subscription-based).

Pokemon in the games had been stored inside cartridges from Gen 1 to Gen 7 (90s to early 2019). It was only with the Switch that game saves (and thus Pokemon) stopped being saved to the cartridges, and instead were saved to the console’s internal storage (not even an SD card).

Until 2019, getting a new console for your Pokemon games meant nothing – you’d be able to use your old Pokemon and saves on the new console by plugging in your old game.

This is the first time you had to do anything special to get your saves onto your new console.

For added context, Nintendo offers a cloud saves for players to back up their Switch game saves.

Pokemon is the one of the only Nintendo franchises to not support cloud saves.